The invention relates to a method for measurement of the profile geometry of spherically curved, particularly cylindrical, bodies and to an apparatus for carrying out the method.
The present invention relates in particular to an optical profile measurement, which is implemented without contact using the laser-section method as a two-dimensional triangulation method known per se.
A three-dimensional “overall profile” of the measurement object, like e.g. a tube, can hereby be constructed by joining the successively captured two-dimensional “profile sections”, when sensor and measurement object move relative to one another.
The profile measurement considered within the context the present invention is in its one-dimensional implementation based on the known point triangulation, wherein a laser and a linear position-sensitive detector form the triangulation sensor. Laser beam axis and optical axis of the detector span a plane referred to hereinafter as “normal plane” and enclose a triangulation angle. The distance of the measurement object from the sensor in the direction of the laser beam normally constitutes the measured variable. This method is known e.g. from DE 40 37 383 A1.
The extension of the point triangulation to two dimensions is the subject matter of the present patent application. In this also generally known light-section method, the point-shaped laser beam is replaced by a laser beam fan and the one-dimensional, linear detector is replaced by a two-dimensional area detector.
In the known method, the extension is implemented orthogonally and symmetrically with respect to the normal plane as designated above. The respective measurement field on the measurement object is imaged onto the detector by an objective, with the objective and detector forming a two-dimensionally operating area imaging camera.
The laser beam fan is typically generated by diffractive optics mounted in front of the point-shaped laser beam exit and thus produces a line on the measurement object referred to as “light-section line”.
When applying the aforedescribed method to cylindrical measurement objects, for example—but not necessarily—tubes, the light-section line is typically oriented perpendicular to the tube axis. When the tubes are transported in direction of a longitudinal axis or when the sensor is moved accordingly, a three-dimensional profile of the tube geometry can be captured in a continuous measurement, as mentioned above.
When applying the light-section measurement in the aforedescribed manner and arrangement, a disadvantage caused by the geometry of the measurement object becomes evident, which even makes to some extent an exact determination of the profile geometry impossible.
FIG. 1 schematically illustrates the conventional two-dimensional light-section measurement of a cylindrical tube. The measurement uses an image of a surface of the measurement object 4 imaged on a detector, which is constructed as a camera 3, wherein the surface is illuminated by a projected fan-shaped laser line 2 emitted from a laser 1.
The image on the left-hand side of the drawing shows schematically the view as a longitudinal section and the image on the right-hand side as cross-section in relation to the longitudinal axis of the measurement object 4. The light-section arrangement of laser 1 and camera 3 are hereby arranged in the normal plane aligned with the longitudinal axis of the measurement object, with the angle enclosed between the axis of the laser beam fan 5 of the laser 1 and the optical axis 6 of the camera 3 being the triangulation angle in longitudinal section.
A disadvantage of this arrangement is that only a relatively small part of the laser beam energy scattered back from the object surface reaches the camera 3 for evaluation. In particular, the energy per exposure time interval is relevant in particular when performing a dynamic measurement, a fact that is critical especially for a rapid relative movement between camera 3 and measurement object 4 and an accompanying necessarily short exposure time and may even make a three-dimensional profile measurement impossible.
When applied to the cylindrical geometry, this disadvantage of the measurement with the known method is amplified by the angle conditions due to the fan-shaped widening of the laser beam 2 and has therefore an impact particularly in the marginal regions of the measurement field, in particular where a still smaller portion of backscattered laser energy is captured by the camera 3 due to the curvature of the surface.
The drop in intensity in the marginal areas of the imaged light-section line is also disadvantageous for the signal/noise ratio in the evaluation of the measurement signals and thus ultimately for the measurement accuracy of the signals.
Although an increase in the output power of the laser generally improves the signal/noise ratio, it disadvantageously increases the complexity of the laser and the laser safety.
Although a possible increase in the exposure time of the camera is principally possible, this is however ruled out when a relative movement between sensor and measurement object is rapid because of the increasing motion blur.
In principle, a steeper viewing angle of the camera (i.e. a steeper triangulation angle) would be conceivable with a typical scattering characteristic of the measurement object surface; however, this would reduce in particular the measurement resolution.
The afore-mentioned three measures would however not be able to solve the problem of uneven intensity distribution during the image acquisition; in the presence of adequate edge intensity, there would even be the risk of overexposing the central region.
The present invention is therefore based on the object to provide an easy-to-implement method for measurement of the profile geometry of a measurement object using two-dimensional light-section method of spherical, in particular cylindrical, bodies, which is able to overcome the described disadvantages. A further object is to provide a corresponding apparatus.